1. Field of the Invention
The present invention relates generally to composite materials that are cured at elevated temperatures and pressures. Typically, such composite materials are cured in an autoclave where temperatures and pressures can be carefully controlled. More particularly, the present invention relates to the polymer resins that are used in the fabrication of these composite materials. The invention involves the preparation and use of liquid heat-settable resin mixtures. The mixtures are formed into curable solid resins that may be heat cured to form composite structures without the need for an autoclave or the application of high consolidation pressures.
2. Description of Related Art
Composite materials are widely used in situations where high strength and low weight are required. Composite materials generally include one or more layers of fibers that are embedded in a resin matrix. Glass and graphite fibers are particularly popular with many other types of fibers being available. Fibers come in an extremely wide variety of sizes, shapes and configurations. Woven fabric, unidirectional fibers, randomly oriented fiber matting and chopped fibers are just a few examples. Although numerous types of matrix resins are available, epoxy resins have been extremely popular.
Composite materials based on the use of graphite or glass fibers in combination with thermosetting polymer resins are frequently found in aerospace and automotive vehicles. These materials are especially well suited for use in aircraft where the combination of high strength and low weight are particularly desirable. Composite materials have been used to produce a wide variety of aircraft parts ranging from non-structural panels and nacelles to structural components including flight control surfaces and structural elements of the wing, fuselage and tail.
A common process for fabricating composite material parts involves first preparing a number of separate fiber layers that are impregnated with an appropriate resin mixture that includes a thermosetting epoxy and one or more curing agents. The resin impregnated fiber layers are referred to as “prepreg”. The prepreg layers are laminated together, typically in a mold, and cured to form the final composite part. Using prepregs to form composite parts is desirable because it allows one to carefully control the amount of resin that is present in the final composite material. Other lamination or lay up procedures include those that involve impregnation of the resin into the dry fiber layers as they are placed in the mold.
The current practice for curing thermosetting composite materials requires that the layers of impregnated fibers be consolidated under high compaction pressures while heating to laminate the individual layers together to form the desired composite part. As mentioned above, curing is usually done using an autoclave where the pressure and temperature can be controlled. Even small autoclaves that are intended for use in curing relatively small composite parts can be very expensive. Larger autoclaves that are designed to handle composite parts found on large aircraft and aerospace launch vehicles are even more expensive.
In order to substantially reduce the cost of fabricating large composite parts, it would be desirable to provide thermosetting composite materials that can be used to fabricate large parts without the need for an autoclave or other expensive processing equipment. Attempts have been made to eliminate the autoclave from the fabrication process. These “out-of-autoclave” procedures have been largely unsuccessful. One process is based on trying to achieve full cure during the lay down step. In this process, the layers are heated and consolidated against underlying layers to form a finally cured product. In another process, the layers are consolidated together in the lay down step with little or no curing reaction taking place. Both of these approaches are difficult to carry out and have not been entirely successful due to difficulties in achieving uniform and consistent properties.